This is NOT for my truck, but A Dodge Truck friend of mine (Clark) has a problem with HIS brakes being "spongy". He has a 1969 D-100 with "standard manual" (not power) brakes. So... after a few of my "ideas" he is still having a problem. So, if YOU GUYS can help out Clark, I'll forward the info to him
Here is his most recent email to me:
Jim:
I have a friend that loves this mechanic here in town. He kept nagging me to "Let Pete have a look at it, Pete will fix it for you", etc. Well, I took the Dodge to Pete. I told him I'd been through a couple of master cylinders and the pedal was still spongy and wasn't returning. He nodded and said "Proportioning valve". I nodded back and told him I'd tracked one down and installed it, and the pedal was still not coming back. He furrowed his brow and told me to leave the truck with him.
I waited a week to call him for an update. He told me he'd taken out the brake pedal and lubed it up and re-installed it. I told him I'd done that already and it didn't help. He said "Something's going on with this truck. I hope you're not in a hurry to get it back."
I waited another week before I checked in with Pete. This time he told me that he'd found a leak in the proportioning valve (actually, what he said was that "it was PUKING fluid") and he and his assistant had bled the system thoroughly, and the brake pedal was "about 100% better" and "You can come and get it if you want to but it's not working right yet." I told him to hang onto it and see if he could figure out what was going on with it.
I waited another week before I called for an update. Pete was clearly pissed off about this brake problem. He asked me "Can you remember exactly when this problem started?" and I said "Yeah... when I changed the master cylinder circa 2000, and the new master cylinder didn't look anything like the original one." He was freaked out by this bit of info. "What do you mean? What was different about the new master cylinder?" "Well, my old master cylinder had a tall cover with two humps and a wire bail that snapped over it to hold it on. The new ones all have a flat lid with a bolt that holds it down." (SEE PHOTOS BELOW)
Pete told me he'd look into that and let me know what he could find out. A few days later, he called me and told me he'd discovered that the older master cylinders had a larger piston (larger "bore") and therefore, moved more fluid faster. These new replacement master cylinders that I'd been buying were "underpowered" for the weight my truck. He told me the old master cylinders had a 1 1/8" piston/bore, while the newer ones were only 1", or a bit under 1".
I did some Googling and found a message board where a few guys were talking about this problem. They had muscle cars, like Barracudas or something, but they did mention trucks. One guy said "It sounds like you need a large bore master cylinder, the kind Dodge made for their trucks. Some car guys don't like the feel of these large bore masters, because the pedal is really hard. It doesn't have any 'give'. A lot of guys, myself included, like the brake pedal to be a little softer. But they will work on your car."
Man, this sure sounds like what I need!
How can I get a large bore master cylinder with the 1969 D100 4-bolt pattern mounting? The only ones I can find online are for "front disc & rear drum" brake systems. My truck has drums all around and NO power assist.
Clark
I was informed that he would be happy with a used one that he could have rebuilt... if anybody has a large bore model with the 4-bolt mounting pattern! Let me know, and I'll pass on the info to him.
We don't really know how good the mechanic is but, it may not be the master. I would troubleshoot the problem first. I would block off the ports to the master and it will be rock hard if good, if not, its not bled or it is bad. Then systematically find out what is giving in or moving. Brake hoses, shoes not radiused, not adjusted correctly or has a leak and air is in the lines. Block one side see if front or rear, then start there. The guy said he already replaced the master...…. Somebody is missing something and no point throwing parts at the issue. Craig
HUGE RED FLAG HERE:
THERE IS NO FACTORY PROPORTIONING VALVE ON A SWEPT!
Thanks Terry (I knew you were going to post that sooner or later ), yeah I know that it isn't a proportioning valve, but that distribution block is just what he called it. I believe I told him the same thing about the "block" in an earlier email to him.
You can run a disc master on drums, just not the other way around. Disc masters are set up to move more fluid because discs require more fluid. They make the same pressure as a drum master because pressure is a function of the bore. They're just set up with a bigger reservoir and the fluid entry point inn different spot to support the disc volume. I just went through this on a Scout project and ended up with a disc/drum master on a drum/drum system. Worked fine. Just get the master with the right bore and don't sweat it.
You can run a disc master on drums, just not the other way around. Disc masters are set up to move more fluid because discs require more fluid. They make the same pressure as a drum master because pressure is a function of the bore. They're just set up with a bigger reservoir and the fluid entry point inn different spot to support the disc volume. I just went through this on a Scout project and ended up with a disc/drum master on a drum/drum system. Worked fine. Just get the master with the right bore and don't sweat it.
How much travel in the pedal before the pushrod engages the master? Too much and the pedal won't return.
How old are the rubber hoses at the front wheels and rear axle? They deteriorate internally over the yrs, swell up and close off so that little fluid can travel thru them.
I just looked in the FSM specs and a D100 uses a 1" bore master, a D200 1.125" bore. Both use 1.125" slaves.
The spec for pedal travel is NONE for a D100 and 1/4" for a D200.
He has a 1969 D-100 with "standard manual" (not power) brakes. So... after a few of my "ideas" he is still having a problem. So, if YOU GUYS can help out Clark, I'll forward the info to him 
Craig
), yeah I know that it isn't a proportioning valve, but that distribution block is just what he called it. I believe I told him the same thing about the "block" in an earlier email to him.